1. Field of the Inventions
The present embodiments generally relate to systems and methods for draining reservoirs, and more particularly, pump assemblies for draining large reservoirs of cryogenic liquids.
2. Description of the Related Art
In the art cryogenic liquids storage, enormous storage tanks have been constructed with permanently installed high-volume pumps. For example, in the art of utility-scale liquid natural gas storage, storage tanks have been constructed with a diameter of approximately the size of half of a city block and with a height of about 175-feet. A schematic illustration of such a tank is illustrated in FIG. 1.
As shown in FIG. 1, a conventional liquid natural gas storage tank 10 includes an outer tank wall 12 including a generally cylindrical sidewall 14, a flat bottom 16, and a domed top 18. The bottom 16 can be placed on the ground or can be suspended above the ground by pylons 20.
Within the outer tank wall 12, an inner vessel 22 is defined by an inner tank sidewall 24 and a bottom wall 26. The sidewall 24 can be generally cylindrical in shape, corresponding to the shape of the outer wall 14. Similarly, the bottom wall 26 can be flat corresponding with the shape of the lower wall 16.
The upper end of the inner vessel 22 is open. A lid assembly 28 typically is suspended from the domed top 18 by a plurality of struts 30. A seal 32 extends between the lid assembly 28 and the sidewall 24 of the vessel 22. As such, the vessel 22 is sealed, and thus can store a fluid therein. In the illustrated tank 10, the fluid within the vessel 22 includes some liquid natural gas LNG and gaseous natural gas GNG above the liquid natural gas LNG.
Between the outer tank wall 12 and the inner vessel 22, insulation typically is disposed. For example, between the lower walls 16, 26, a rigid insulation 34 typically is disposed. Additionally, a lighter or fluffier insulation 36 can be disposed between the lateral walls 14, 24. Additional insulation can be disposed within the lid assembly 28. Insulated as such, the tank 10 can better maintain the fluid within the vessel 22 at the desired temperature. In the art of the storage of cryogenic liquids, it is desirable to maintain the fluid at a temperature at which the liquid state of the liquid can be maintained. For example, with liquid natural gas LNG, the vessel 22 can be maintained at approximately −260° F. or lower. Other substances can be maintained in a liquid state at other temperatures.
As noted above, tanks such as the tank 10 are often extremely large. Additionally, such cryogenic liquids cannot be vacuumed out of such a tank. This is because when such a liquid is subject to a large vacuum, the liquid boils and therefore will not travel up a vacuum pipe and out of such a tank. Additionally, it is generally undesirable to provide a drain pipe at the bottom of such a tank 10. If such a drain pipe were to fail, enormous amounts of liquid material, such as liquid natural gas LNG, could spill out of such a tank 10, and thereby cause a dangerous situation. Thus, tanks such as the tank 10 typically include a pump 40 mounted near the bottom of the vessel 22 with a discharge of the pump 40 extending upwardly out of the domed top 18. In the illustrated arrangement, the discharge pipe 42 is illustrated schematically and extends to a discharge nozzle 44 above the domed top 18.
In order to provide a reasonable discharge speed of the liquid natural gas LNG, the pump 40 is quite large in size and has a high horsepower rating. Additionally, the motor 40 must be sealed and be made from a proper material to be operated in the liquid environment of the liquid natural gas LNG and at the environmental temperature of approximately −200 F. Typically, the motor 40 is suspended by the discharge pipe 42. Thus, as noted above, because the tank can be approximately 175 ft. tall, the discharge duct 42 is made from a thick, high strength material that is appropriate for a cryogenic environment. For example, the discharge pipe 42 can be made from stainless steel or aluminum.
As illustrated in FIG. 1, the discharge pipe 42 has a lower portion that can be submerged below the level of the liquid natural gas and an upper portion, adjacent the discharge nozzle 44, that is exposed to the atmosphere. Thus, the discharge pipe 42 is subject to substantial expansion, contraction, as well as thermal stresses. In order to prevent the discharge pipe 42 from contacting the lower surface 26 of the vessel 22, a clearance C is defined between the lower end of the discharge pipe 42 and the lower wall 26. In many typical tanks such as the tank 10, the clearance C can be as much as 18 to 24 inches or more.
The tank 10 also includes an instrumentation assembly 50. The instrumentation assembly 50 includes an instrument guide duct 52 extending through the domed top 18 and the lid assembly 28 into the vessel 22, a valve 54, an instrument head 56, and at least one instrument 58 configured to detect a state of the material within the vessel 22.
The instrument guide tube 52 can be made from any material. However, typically, the instrument guide tube 52 is made from a stainless steel pipe having an inner diameter of between 5-½ inches and 10 inches. The instrument 58 is suspended from the instrument head 56 by a cable 60. The instrument head 56 can include a winch 62 configured to raise and lower the instrument 58 through the instrument guide tube 52. The valve 54 can be configured to allow the instrument 58 to be retracted entirely into the instrument head 56. For example, the valve 54 can be a “gate” type valve. With such a valve, when the valve is open, the passage extending through the valve 54 is completely open through the entire bore through the valve 54. Alternatively, the valve 54 can be a butterfly-type valve. With a butterfly-type valve, when such a valve is open, the pivot shaft and valve plate remain within the bore of the valve 54, thereby partially obstructing the passage therethrough.
When a tank such as the tank 10 reaches the end of its useful life, it is typically emptied of liquid natural gas LNG and subsequently decommissioned and/or disassembled. Initially, the liquid natural gas LNG will be pumped out of the vessel 22 by the existing pump 40. However, as noted above, the resulting clearance C prevents the pump 40 from reaching residual liquid natural gas RLNG at the bottom of the vessel 22. Because the clearance C can be large, as noted above, the volume of residual liquid natural gas RLNG can be quite large.
One way to remove the residual liquid natural gas is to allow it to evaporate out of the tank through existing plumbing. Typically, it can take approximately three months to allow such a volume of residual liquid natural gas LNG to evaporate out of the tank 10. Additionally, such an evaporation process must be monitored to ensure public safety. Thus, the process of decommissioning a tank, such as the tank 10, can be a long process.